Boilers for electric power generation often have combustion systems with furnaces that are fired with solid fuel, such as coal, lignite, etc.; these combustion systems are usually provided with mills for pulverizing the fuel and ducting for supplying the pulverized fuel to burners of the furnace.
In these boilers, both fuel quality and achievable dust concentration influence operational flexibility, safe ignition, and flame stability.
In particular, in case of lignite fired boilers, fuel concentration is an important parameter to control, because of the very different features of different kinds of lignite, such that in order to maintain safe operation it is necessary to increase pulverized fuel concentration when the quality of the lignite lowers.
In order to increase fuel concentration it is common the use of the so called vapour separation systems; these systems separate the flow coming from the mill in a fuel rich flow and direct it to burners located at a lower zone of the furnace and a fuel lean flow (i.e. a vapour rich flow) and supply it to burners located at an upper zone of the furnace.
Different vapour separation systems have been proposed.
A first example of vapour separation system takes advantage of the non-homogeneous flow coming from the mill. In this case a branching in the duct that carries the flow from the mill causes separation of the flow in a fuel rich flow in one ducting and fuel lean flow in other ducting.
This vapour separation system proved to cause low pressure losses while ensuring good separation performances.
A different example of vapour separation system provides for an impeller that divides a homogeneous flow between different ducting; in particular the impeller forces separation of a fuel rich flow from a fuel lean flow and directs each flow in different ducting. For example, DE 293 35 28 discloses a vapour separation system of this kind.
This vapour separation system proved to be very effective in separation, but at the same time it causes high pressure losses.
Lignite fired boilers have to guarantee a broad operation load range but, because of the intrinsic features of the lignite, at low load (for example load below 50%, preferably 40%, more preferably 30%, and even more preferably below 20%) the fuel concentration achievable with the known vapour systems and/or the pressure losses cannot guarantee safe operation.